1. Field of the Invention
The invention relates to a display-image switching apparatus for a vehicle, and in particular the invention relates to a display-image switching apparatus that switches a display image between an image captured by a camera provided in a vehicle to monitor the surroundings of the vehicle and another image, in accordance with the speed of the vehicle.
2. Description of the Related Art
Japanese Patent Application Publication No. 5-238313 (JP-A-5-238313) describes an in-dash information display apparatus that selectively shows an image output from a TV tuner or a navigation system and an image output from a vehicle camera in a display (e.g., CRT) provided in the passenger compartment of the vehicle. According to this apparatus, for the purpose of improving user-friendliness, the image shown in the display is switched in accordance with the speed of the vehicle. More specifically, the image of the navigation system or the TV tuner is shown in the display when the vehicle speed exceeds a threshold speed (e.g., when the vehicle is running at a normal speed), and the display image is automatically switched to the image of the vehicle camera when the vehicle speed falls to or below the threshold speed (e.g., when the driver should pay much attention to the circumference, such as during low-speed driving of the vehicle).
However, the above-described display-image switching apparatus involves the following problems. First, the accuracy in determining the vehicle speed is limited. Typically, the vehicle speed is estimated from the rotational speed of the wheels of the vehicle. However, the wheel speeds fluctuate significantly if an ABS (Anti-Lock Brake System) is operating, for example. That is, for example, upon panic braking, the ABS controls the slip rates of the respective wheels within desired ranges so as to avoid locking of the wheels. Under such circumstances, the wheels are locked and released alternately in very short cycles, thereby causing large fluctuations in the wheel speeds.
The graph in FIG. 6 illustrates how the vehicle speed determined from the wheel speeds (will be referred to as “wheel-speed-based vehicle speed”) changes during operation of ABS. The horizontal axis of FIG. 6 represents time, and the vertical axis represents the wheel-speed-based vehicle speed. Referring to FIG. 6, although the wheel-speed-based vehicle speed 901 generally decreases in time, it goes through several sharp increases and decreases. That is, the wheel-speed-based vehicle speed fluctuates significantly during operation of ABS. Therefore, if the display image is switched based on the wheel-speed-based vehicle speed as described in JP-A-5-238313 while the vehicle speed is fluctuating during operation of ABS, the display image is switched unnecessarily, and frequently, and this is not desirable. The graph of FIG. 7 illustrates an example where display-image switching control is executed based on a change in the wheel-speed-based vehicle speed. Referring to FIG. 7, the navigation image is shown in the display when the wheel-speed-based vehicle speed 901 exceeds a threshold speed 903, and the vehicle camera image is shown in the display when the wheel-speed-based vehicle speed is below the threshold speed 903. In this example, the display image is repeatedly switched between the navigation image and the vehicle camera image in short cycles. Thus, during operation of ABS, due to such repeated display-image switching between the navigation image and the vehicle camera image, the display image becomes hard to recognize (it appears like a flickering image). That is, undesired display image switching occurs frequently.